1. Field of the Invention
The invention relates generally to fuses, and more particularly, to a full range current limiting fuse, that is to a current limiting fuse that can interrupt any current shown on its published minimum melt time-current curves.
2. Description of the Prior Art
Current limiting fuses conventionally comprise a fusible element embedded in a granular inert material of high dielectric strength such as sand or finely divided quartz. Usually the fusible element is in the form of one or more thin conductors of silver wound on a supporting insulating core or spider. When subjected to current of fault magnitude, the fusible element attains fusing temperature and vaporizes, whereby arcing occurs and the metal vapors rapidly expand to many times the volume originally occupied by the fusible element and are thrown into the spaces between the granules of inert filler material where they condense and are no longer available for current conduction. The current limiting effect results from the interaction of the metal vapors and the inert granular material surrounding the fusible element. The physical contact between the hot arc and the relatively cool granules causes a rapid transfer of heat from the arc to the granules, thereby dissipating most of the arc energy with very little pressure build-up within the fuse enclosure. The vapors of silver have relatively low conductivity unless their temperature is particularly high, and the temperature of the silver vapors is rapidly reduced by the quartz-sand filler until the vapors will not support a flow of current. Consequently, a high resistance is, in effect, inserted into the path of the current and initially limits the current to a magnitude which is only a small fraction of that available in the circuit.
The quartz sand particles in the immediate vicinity of the arc fuse become partial conductors at the high temperature of the arc and form a fulgurite, or semi-conductor. The fulgurite resulting from fusion and sintering of the quartz sand particles is in the nature of a glass body, and as it cools it loses its conductivity and becomes an insulator.
High voltage, high amperage current limiting fuses conventionally employ fusible elements of silver ribbon having serially related portions of relatively small cross sectional area and intermediate portions of relatively large cross sectional area, for example, a silver ribbon provided with a plurality of circular spaced apart perforations which determine the portions where the fusion of the fusible element is initiated on currents of short circuit magnitude. The perforations form portions of reduced cross sectional area which limit the peak arc voltage and make it possible to distribute the thermal duty of the arc quenching granular material relatively evenly over the entire filler body.
If such a fuse is subject to fault currents of high magnitude, all the portions of small cross sectional area fuse and vaporize almost simultaneously, resulting in formation of arclets in series and controlling the transient voltage across the fuse.
These fusible ribbons generally include a "M" spot, that is, a body of low melting temperature alloy such as tin-lead solder in intimate contact with the ribbon adjacent the midpoint thereof, to assure that on fault currents of low magnitude, the first arc gap will be formed near the middle of the fuse. At melting currents flowing for prolonged periods, the fusible ribbons become hot enough to melt the alloy bodies, and the amalgamation of the silver and alloy causes a hot spot with high enough resistance to melt the ribbon at this point. However, on large magnitude fault currents, the alloy element has little or no effect and the silver elements vaporize at the fusion temperature for the silver.
When the fuse is subjected to low magnitude overload currents, the arc gap first formed at the "M" spot is generally progressively enlarged by vaporization of the silver element until the gap is of sufficient length to effect final interruption of the circuit and consequently the fulgurite produced by the arcing is generally continuous. When such interruption of small overload currents result in arcing over a plurality of of cycles, the arc energy tends to be large. The relatively large arc energy and the dissipation of additional heat resulting from I.sup.2 r losses caused by the flow of follow current through the fulgurite combine to delay the cooling of the central portion of the fulgurite which remains partially conductive, and only the end portions of the fulgurite, where the arc contacts are relatively cool filler particles, tend to interrupt the arc. Most of the voltage appears across the ends of the fulgurite, which are of higher resistance and the hot central portions thereof, and tends to flash over the hot gases, and consequently reignition of the fusible element and post-interruption failure can occur when current limiting fuses of this type control overload currents of small magnitude.
In order to produce additional points of arcing in the fusible element during protracted low magnitude overload currents, some or all of the portions of reduced cross sectional area of the fusible element can be designed to be melted by the heating resulting from the I.sub.2 r losses occurring in these portions when a low magnitude overload current flows therethrough. However, such a drastic reduction in the cross sectional areas or lengths of these portions of the main element greatly reduces the transient surge currents that the fuse can withstand.
In the current limiting fuse disclosed in U.S. Pat. No. 3,243,552 issued Mar. 29, 1966, to Harvey W. Mikulecky, not only are additional arcing points established in the main fusible element by means other than the I.sup.2 r loss through the element, but also the first arcing point at the "M" spot is temporarily extinguished to allow this portion of the fulgurite to cool and become a nonconductor. This is accomplished by the use of an auxiliary fusible element having its ends closely and accurately spaced from the main fusible element on opposite sides of the "M" spot, and having a minimum melting current sufficiently less than that of the main fusible element so that, when the minimum melt current is reached for the main element, good low current clearing characteristics exist for the auxiliary element. When this fuse is subjected to a low magnitude overload or fault current, the main fusible element opens at its "M" spot and starts to arc and burn back. When the arc voltage crossing this area is high enough, the auxiliary gaps are sparked over, resulting in the auxiliary element becoming the path for the current and extinguishing the arc at the "M" spot, allowing the fulgurite at the "M" spot to cool and lose its conductivity. While the arc exists at the auxiliary gaps they cut through and burn back the main ribbon element. The auxiliary element then clears the circuit and the arcs at the gaps go out. If not enough of the main element has been consumed to withstand the recovery voltage across the fuse, the gaps in the main ribbon element at the auxiliary locations and the "M" spot restrike and burn back until a sufficient dielectric path has been established to withstand the recovery voltage.
However, some of the fuse ratings of this design of current limiting fuse have a particularly hard interruption duty at low magnitude fault overload currents because of the long arcing times, up to 100 cycles, required for the fuse to clear. These long arcing times release large amounts of arc energy at discrete locations in the fuse which can thoroughly damage the fuse components. Also, when the fusible element is wound about a spider of a material which evolves gas in the presence of an arc for cooling the inert granules, as also disclosed in the above referenced U.S. Pat. No. 3,243,552, the excessively long arcing times can cause an excessive amount of gas generation in the fuse. When this fuse is used in tight containers, this gas can escape from the fuse and condense on adjacent dielectric materials to cause a flashover of these materials.